According to a known constructional feature employed in the automobile engines with bucket type tappets, for ensuring the correct performance of the opening and closure phases of valves there is foreseen the insertion--in the valve cups--of shims that have such a thickness as to provide a predetermined amount of clearance between the shims and the base circles of the cams.
Conventional gauges are also known, which determine the thickness of the shims in the course of the assembly of an engine, by measuring the clearances when the camshaft and the valves are already assembled. According to this method there are measured the lifts of the valves and relevant cams during a complete rotation of the camshaft and the thicknesses of the shims are determined by calculating the differences between the lifts of the valves and the lifts of the corresponding cams. In these known gauges the valve lifts are detected by feelers that cooperate with the bases of the valve heads and the lifts of the cams by other feelers, aligned with the previous ones, that cooperate with the surfaces of the cams.
A considerable inconvenience occuring in these known gauges derives from the fact that under normal circumstances the bases of the valve heads are not machined with accuracy and consequently are not reliable references.
A further inconvenience is due to the fact that these gauges can be applied only when the engine is in an assembling phase according to which the bases of the valve heads are accessible; in other words, before assembling of the cylinder head on the engine block.